1. Field
Example embodiments of the present invention relate to a nonvolatile memory device having two resistance elements, and more particularly, to a nonvolatile memory device formed and operated using a resistance element having memory switching characteristics and a resistance element having threshold switching characteristics.
2. Description of the Related Art
A good semiconductor memory device must have a high integration density, which means that the number of memory cells per unit area is high, a high operation speed and/or a drive capability at low power. Thus, many efforts have been made to develop such a semiconductor device, and various kinds of memory devices are produced.
Generally, a semiconductor memory device includes many memory cells connected as circuits. In the case of a dynamic random access memory (DRAM) as a typical semiconductor memory device, a unit memory cell is normally composed of one switch and one capacitor. DRAMs have advantages of higher integration density and a higher operation speed. However, a DRAM's stored data is lost when power is cut off.
Conversely, a typical example of a nonvolatile memory device capable of storing data even with power cut off is a flash memory device. A flash memory device is nonvolatile, unlike a volatile memory device, but may have detriments of a lower integration density and/or a lower operation speed, as compared to a DRAM.
Recently, the types of nonvolatile memory devices under development may include a magnetic random access memory (MRAM), a ferroelectric random access memory (FRAM), and a phase-change random access memory (PRAM).
An MRAM stores data using change of magnetization direction at a tunnel junction and a FRAM stores data using polarity property of ferroelectrics. These devices have their own advantages and disadvantages, and mostly are studied and developed toward higher integration density, higher operation speed, a drive capability at lower power, and an improved data retention property.
A PRAM stores data using change of resistance values in accordance with phase change of specific materials, and has one resistance material layer and one switch (for example, a transistor). The resistance material layer used in the PRAM may be a chalcogenide resistor, which may have a crystal structure or an amorphous structure a formation temperature. Because a resistance in an amorphous state is normally higher than in a crystal structure, a memory device may be fabricated using this property. When a conventional fabrication process of a DRAM is employed to fabricate a PRAM, etching is difficult and it may take a long time. Further, conventional memory devices may have a problem of achieving a precise switching operation because its structure is complicated when switching using a transistor or diode.